Since the installation in dental handpieces and dental air/water syringes of anti-retraction valves to prevent back-contamination by patients' saliva, the main source of microbial contamination of dental waterlines is in the source water. Most jurisdictions provide mains tap water with less than the American Dental Association guideline of 200 CFUs/mL, but the low flow rates of water used in most dental procedures and the small bores of dental unit waterlines allow the build up of biofilm in dental waterlines. Thus counts of 10,000-100,000 CFU/mL are not uncommon in water leaving the cooling water nozzle of high-speed dental drills and from dental air/water syringes. Even when sterile water is delivered to bottled-water-fed dental units, it is not unusual for stray contaminating bacteria to enter the waterlines and multiply exponentially with the result of CFU counts the same as in tap-water-fed dental units. In some circumstances, bacteria are able to replicate every 20 minutes, which means one bacterium left in a waterline after the last treatment at, for instance 5:00 pm could have multiplied to over 2 million by midnight.
Even though the potential flow-rate of water through most dental unit waterlines to the high-speed drill is in the order of 60 mL per minute, most drilling operations require flow-rates much below this for adequate cooling and only a small portion of each patient's treatment actually involves drilling. Thus, ample opportunity occurs for bacterial accumulation even during office hours and very little actual flushing occurs. This means that even if water treatment devices such as the ozonizer described in U.S. Pat. No. 5,942,125 are installed, the contact time between ozone and the harmful biofilm is limited and restricted to office hours. Ozone in water decays to oxygen within 20-30 minutes and so any bacteria surviving the brief contact time and ozone decay have ample time to replicate.
The system described in U.S. Pat. No. 6,482,370 attempts to address this problem by installing a tee piece at the end of the waterline closest to the dental instrument (such as high-speed drill or air/water syringe) attached to a return tube that recirculates ozonized water back to the ozonizer. Unfortunately, such a tee piece and return tube constitute a considerable modification to hardware and plumbing of existing dental chairs and waterlines. In addition, during recirculation of the ozonized water, the ozonized water is not fed through the delivery structure for the dental instrument and any residual bacteria have the opportunity to multiply and contaminate the delivery structure so that when a freshly sterilized dental instrument is coupled to the dental water line, it too becomes contaminated. Meanwhile, while the recirculating ozonized water may kill bacteria, their toxic remains such as lipopolysaccharide (lps) are not purged from the system until a dental instrument is connected.
The present invention has been developed to alleviate these drawbacks.